The present invention relates, in general, to problems encountered in scanning systems which ascertain the density of the extreme-density scanned spot on each of a succession of photographic originals transported through the scanning system.
Most especially, the present invention relates to problems encountered with the scanning system disclosed in the commonly owned U.S. patent identified above. In that system, a light source transmits light, through each original in a continuously transported strip of originals, to a light detector. A rotary scanning disk is located in the path of this light and provided with equiangularly spaced radial scanning slots. A light-blocking plate is located between the light source and the scanning disk and is provided with a scanning slot which extends transverse to the transport direction of the strip. Light which passes through both an original at the scanning station and also the transverse scanning slot is projected as a slot image onto the surface of the scanning disk. The radial slots and the transverse slot cooperate to define a scanning spot which sweeps across the original, and thereby performs one line-scan operation. In particular, one line-scan operation is performed as one radial scanning slot sweeps across the slot image projected onto the surface of the rotary scanning disk. The dimensions of the scanning slots and the spacing of the radial scanning slots are such that the scanning spot defined by them is of approximately constant configuration throughout the course of the line-scan operation. An evaluating circuit receives the signal produced by the light detector and determines the density of the extreme-density scanned spot on each scanned original, for example the density of the maximum-density scanned spot on each scanned original.
With this scanning system, and with other scanning systems employing a bright lamp or other such light source for generating scanning light, use must be made of a light source whose output light intensity is extremely constant, both short-term and long-term, and the evaluating circuitry of the scanning system must be adjusted for correspondence with the output light intensity of the light source. However, lamps of this type in general provide an output light intensity which is not sufficiently constant and reliable. This is attributable to simple aging of the lamps, power-supply variations, differences between one lamp and another when a lamp is replaced, and other such factors. The extent to which the output light intensity of the lamp may vary detracts from the accuracy of the extreme-density-value measurement. Certainly, when one lamp is replaced by another, the entire system must be recalibrated.